Genetic and environmental influences on the development of prosocial and antisocial behavior is studied in monozygotic and dizygotic twins. This is part of a larger longitudinal collaborative investigation based in Colorado, of twins studied from late infancy through 7 years of age. Because prosocial and antisocial patterns are regulated, in part, by emotions such as empathy and anger, the etiology of these corresponding emotion states also is investigated. Children and their mothers are observed in structured and naturalistic situations in both laboratory and home settings. Maternal observational reports also are obtained. In one current project, children's responses to others' distress are examined in MZ and same-sex DZ twins at 14, 20, 24, and 36 months of age. Genetic influence on prosocial patterns (evidenced in cross-twin correlations and regressions) is present at each time point for affective, behavioral and cognitive expressions of concern for others. Patterns of heritability also are identified across time, typically at adjacent time points. Prosocial acts, empathic concern, and hypothesis-testing increase with age, while self distress declines. Boys show more indifference than girls to the distress of others, whereas girls show more concern. A second project focuses on the early presence and development of anger propensity (at 14, 20, and 24 months). While current evidence suggests context- specificity rather than a generalized trait, genetic influence on anger expression in very young children is present in each of the specific contexts examined.